Unless otherwise indicated herein, the materials described in this section are not prior art to the claims and are not admitted to be prior art by inclusion in this section.
In a wireless communication system, a base station may provide one or more coverage areas, such as cells or sectors, in which the base station may serve wireless communication devices (WCDs), such as cell phones, wirelessly-equipped personal computers or tablets, tracking devices, embedded wireless communication modules, or other devices equipped with wireless communication functionality. In general, each coverage area may operate on one or more carriers each defining a respective bandwidth of coverage, and each coverage area may define an air interface providing a downlink for carrying communications from the base station to WCDs and an uplink for carrying communications from WCDs to the base station. The downlink and uplink may operate on separate carriers or may be time division multiplexed over the same carrier(s). Further, the air interface may define various channels for carrying communications between the base station and WCDs. For instance, the air interface may define one or more downlink traffic channels and downlink control channels, and one or more uplink traffic channels and uplink control channels.
In accordance with the Long Term Evolution (LTE) standard of the Universal Mobile Telecommunications System (UMTS), for instance, each coverage area of a base station may operate on one or more carriers spanning 3 MHz, 5 MHz, 10 MHz, 15 MHz, or 20 MHz. On each such carrier used for downlink communications, the air interface then defines a Physical Downlink Shared Channel (PDSCH) as a primary channel for carrying data from the base station to WCDs, and a Physical Downlink Control Channel (PDCCH) for carrying control signaling from the base station to WCDs. Further, on each such carrier used for uplink communications, the air interface defines a Physical Uplink Shared Channel (PUSCH) as a primary channel for carrying data from WCDs to the base station, and a Physical Uplink Control Channel (PUCCH) for carrying control signaling from WCDs to the base station.
In LTE, downlink air interface resources are mapped in the time domain and in the frequency domain. In the time domain, LTE defines 10 millisecond (ms) frames, 1 ms sub-frames and 0.5 ms slots. Thus, each frame has 10 sub-frames, and each sub-frame has 2 slots. In the frequency domain, resources are divided into groups of 12 sub-carriers. Each sub-carrier is 15 kHz wide, so each group of 12 sub-carriers occupies a 180 kHz bandwidth. The 12 sub-carriers in a group are modulated together, using orthogonal frequency division multiplexing (OFDM), to form one OFDM symbol.
LTE further defines a particular grouping of time-domain and frequency-domain resources as a downlink resource block. In the time domain, each downlink resource block has a duration corresponding to one sub-frame (1 ms). In the frequency domain, each downlink resource block consists of a group of 12 sub-carriers that are used together to form OFDM symbols. Typically, the 1 ms duration of a downlink resource block accommodates 14 OFDM symbols, each spanning 66.7 microseconds, with a 4.69 microsecond guard band (cyclic prefix) added to help avoid inter-symbol interference. Depending on the bandwidth of the downlink carrier, the air interface may support transmission on a number of such downlink resource blocks in each sub-frame. For instance, a 5 MHz carrier supports 25 resource blocks in each 1 sub-frame, whereas a 15 MHz carrier supports 75 resource blocks in each 1 sub-frame.
The smallest unit of downlink resources is the resource element. Each resource element corresponds to one sub-carrier and one OFDM symbol. Thus, a resource block that consists of 12 sub-carriers and 14 OFDM symbols has 168 resource elements. Further, each OFDM symbol and thus each resource element can represent a number of bits, with the number of bits depending on how the data is modulated. For instance, with Quadrature Phase Shift Keying (QPSK) modulation, each modulation symbol may represent 2 bits; with 16 Quadrature Amplitude Modulation (16QAM), each modulation symbol may represent 4 bits; and with 64QAM, each modulation symbol may represent 6 bits.
Within a resource block, different resource elements can have different functions. In particular, a certain number of the resource elements (e.g., 8 resource elements) may be reserved for reference signals used for channel estimation. In addition, a certain number of the resource elements (e.g., resource elements in the first one, two, or three OFDM symbols) may be reserved for the PDCCH and other control channels. In each sub-frame, the resource elements that define these control channels cooperatively span the entire bandwidth, leaving most of the remaining resource elements in each resource block for use to define the PDSCH.
One of the main functions of the PDCCH is to carry “Downlink Control Information” (DCI) messages to served WCDs. LTE defines various types or “formats” of DCI messages, to be used for different purposes, such as to indicate how a WCD should receive data in the PDSCH of the same sub-frame, or how the WCD should transmit data on the PUSCH in an upcoming sub-frame. For instance, a DCI message in a particular sub-frame may schedule downlink communication of bearer data to a WCD, by specifying one or more particular resource blocks that carry the bearer data in the sub-frame's PDSCH, what modulation scheme is used for the downlink transmission, and so forth.
Each DCI message may span a particular set of resource elements on the PDCCH (e.g., one, two, three, or four control channel elements (CCEs), each including 36 resource elements) and may include a cyclic redundancy check (CRC) that is masked (scrambled) with an identifier (e.g., cell radio network temporary identifier (C-RNTI)) assigned to the WCD, so that the WCD can identify and read the DCI message. In practice, a WCD may monitor the PDCCH in each sub-frame in search of a DCI message destined to the WCD. In particular, the WCD may engage in a “blind decoding” process in which the WCD reads various candidate groups of resource elements on the PDCCH in search of a DCI message masked with the WCD's identifier. If the WCD finds such a DCI message, the WCD may then read that DCI message and proceed as indicated. For instance, if the DCI message schedules downlink communication of bearer data to the WCD on particular PDSCH resources in the current sub-frame, the WCD may then read the indicated PDSCH resources to receive that bearer data.
LTE also supports uplink control signaling on the PUCCH using “Uplink Control Information” (UCI) messages. UCI messages can carry scheduling requests from WCDs, requesting the base station to allocate PPUSCH resources for uplink bearer data communication. Further, UCI messages can carry Hybrid Automatic Repeat Request (HARQ) messages from WCDs to the base station, to inform the base station whether downlink data transmissions were successful or not, and to facilitate re-transmission when appropriate. In practice, when the base station schedules downlink communication of bearer data to a WCD on particular PDSCH resources, the WCD may seek to read the data from the indicated resources. If the WCD successfully receives the data, the WCD may then responsively transmit to the base station a UCI message providing an HARQ “ACK” to inform the base station that the WCD successfully received the data. Whereas, if the WCD does not successfully receive the data, the WCD may responsively transmit to the base station an HARQ “NACK,” to inform the base station that the downlink transmission was unsuccessful and to trigger re-transmission by base station.
In a system arranged as above, when a WCD enters into coverage of a base station, the WCD may engage in attach signaling with the base station, by which the WCD would register to be served by the base station on a particular carrier (perhaps a particular pair of downlink carrier and uplink carrier). Through the attach process and/or subsequently, the base station and supporting LTE network infrastructure may establish for the WCD one or more bearers, essentially defining logical tunnels for carrying bearer data between the WCD and a transport network such as the Internet. Each such bearer may have a particular class of service defined by a quality of service class identifier (QCI), for carrying a particular class or type of data. For instance, one such bearer may be a best-efforts bearer for carrying general Internet traffic, another such bearer may be an Internet Multimedia System (IMS) signaling bearer for carrying voice over Internet Protocol (VoIP) setup signaling, and another such bearer may be a guaranteed-bit-rate bearer for carrying VoIP traffic.
Once attached with the base station, a WCD may then operate in a “connected” mode in which the base station may schedule data communication to and from the WCD on the WCD's established bearer(s). In particular, when the base station receives bearer data for transmission to the WCD, the base station may schedule particular PDSCH resources to carry that bearer data and may provide a DCI message to the WCD directing the WCD to receive the bearer data on those PDSCH resources. Similarly, when the WCD has bearer data to transmit to the base station, the WCD may transmit a scheduling request to the base station, and the base station may then schedule particular PUSCH resources to carry that bearer data and provide a DCI message to the WCD directing the WCD to transmit the bearer data on those PUSCH resources.
In a basic LTE arrangement, when a WCD is attached with a base station on a particular carrier (e.g., pair of downlink carrier and uplink carrier), the base station provides DCIs to the WCD on the PDCCH of that carrier and schedules downlink communication of bearer data to the WCD on the PDSCH of that carrier. As a base station may be serving many WCDs at a time, however, the PDSCH on such a carrier may become congested. To help overcome this issue, a revision of LTE known as LTE-Advanced now permits a base station to serve a WCD with “carrier aggregation,” by which the base station schedules bearer communication with the WCD on multiple carriers at a time. With carrier aggregation, a base station may provide a DCI message to a WCD on the carrier on which the WCD is attached (the WCD's “primary carrier” or “PCell”), but may use that DCI message to schedule downlink communication of bearer data to the WCD on two or more carriers at time, such as on both the PDSCH of the WCD's primary carrier and the PDSCH of one or more other carriers (the WCD's “secondary carriers” or “SCells”). Such carrier aggregation can significantly increase the base station's effective bandwidth, well beyond the 20 MHz limit.